feat(TRUEREF-0020): add embedding profiles, default local embeddings, and version-scoped semantic retrieval

- Add embedding_profiles table with provider registry pattern
- Install @xenova/transformers as runtime dependency
- Update snippet_embeddings with composite PK (snippet_id, profile_id)
- Seed default local profile using Xenova/all-MiniLM-L6-v2
- Add provider registry (local-transformers, openai-compatible)
- Update EmbeddingService to persist and retrieve by profileId
- Add version-scoped VectorSearch with optional versionId filtering
- Add searchMode (auto|keyword|semantic|hybrid) to HybridSearchService
- Update API /context route to load active profile, support searchMode/alpha params
- Extend MCP query-docs tool with searchMode and alpha parameters
- Update settings API to work with embedding_profiles table
- Add comprehensive test coverage for profiles, registry, version scoping

Status: 445/451 tests passing, core feature complete

src/lib/server/embeddings/factory.ts

@@ -1,5 +1,9 @@
 /**
  * Factory — create an EmbeddingProvider from a persisted EmbeddingConfig.
+ *
+ * This module maintains backward compatibility with the old enum-style config
+ * while the registry pattern is adopted. Settings endpoints transition to
+ * using embedding_profiles table + registry.ts directly.
  */
 
 import type { EmbeddingProvider } from './provider.js';
@@ -7,6 +11,9 @@ import { NoopEmbeddingProvider } from './provider.js';
 import { OpenAIEmbeddingProvider } from './openai.provider.js';
 import { LocalEmbeddingProvider } from './local.provider.js';
 
+// Re-export registry functions for new callers
+export { createProviderFromProfile, getDefaultLocalProfile, getRegisteredProviderKinds } from './registry.js';
+
 export interface EmbeddingConfig {
 	provider: 'openai' | 'local' | 'none';
 	openai?: {